In typical urban power distribution systems, access to groups of underground cables is available at manholes which may be rather widely spaced. There may be many cables grouped in any particular manhole and, if work is to be performed on a specific cable of such a group, that cable must-be identified and de-energized before the work proceeds. Some simple methods of cable identification are practiced commonly and these include the use of tags or markers, the noting of manhole rack positions and comparison of ducts in adjacent manholes.
In those situations where terminations are accessible, a coded signal or one of special frequency can be applied to the cable and used to identify that cable at a remote position. This method is not available, of course, where cable terminations are totally inaccessible or sealed and insulated in the manhole. Even when a signal can be applied, misidentification may still occur because inexperienced operators may be confused by signal runoff to the sheath of adjacent cables. Most important, even if cable identification is successful, despite the problems noted, absolute proof that the cable is in de-energized condition is still lacking.
Cables of the type here under consideration usually include a central conductor and an outer metallic sheath separated by composite intermediate layers. Typically, a stranded copper center conductor is embedded in a high-resistivity screen layer which is surrounded by an insulating layer of rubber. A second high-resistivity screen layer is formed around the rubber insulating layer, usually by spiral-wound tape and an outermost layer, typically of lead, forms the metallic sheath about the tape-wound screen layer.
Positive proof that a cable is de-energized invariably requires making an opening at least in the outer sheath. When, for example, a cable is to be spliced, a straight-forward simple test is to spear or cut the cable using a remote, hydraulically actuated ram. During cable penetration, personnel remain at a distance from the test point and are thus protected, but if it turns out that the cable is energized, an outage will result, and frequently serious damage will be done to the cable. The outage and ensuing expensive and time-consuming repairs which are then required are simply unacceptable.
With lead-sheathed cables, it is sometimes the practice to strip back the lead to expose the underlying shield tape. The shield tape may also be cut away to expose the underlying rubber insulation from which the status of the cable may be determined by a capacitively coupled probe. Should it prove to be "hot", the cable must be taken out of service, the damage to the cable must be repaired, and the process repeated until proper identification is made and the original project may be carried out.
Clearly, current methods of cable identification and cable status determination are time-consuming and expensive in terms of work involved and unintentional damage. Moreover, they frequently require the services of highly trained and skilled operators. Finally, in certain situations, there is considerable risk of injury to the operator of the cable testing equipment.
It is therefore a primary object of the present invention to provide an automatic remotely controlled machine and process for the non-destructive unambiguous determination of the status of distribution cables.
Another object of the present invention is to enable relatively unskilled and inexperienced operators to identify and determine the on/off status of underground distribution cables.
It is a further object of the invention to reduce the cost and avoid the damage associated with current methods of cable identification and status determination.